The reason minimum length is stipulated with some power cords is to have enough resistance in the cord to drop noise across it. Understand how a voltage divider works? For argurment’s sake, let’s say the RF choke of your power supply input stage has a reactive impedance of 10 ohms at 100khz and your power cord has a resistance of 10 ohms also. The RFI noise will be equally divided, 50% across the cord’s resistance and 50% across the RF choke on the power supply input. If we make that power cord much shorter, say half as long, we reduce its resistance to 5 ohms while the RF choke reactive impedance stays the same at 10 ohms. V(choke)=V(in)*[Z(choke)/Z(choke)+R(powercord)] so calculating for 100volts RFI, V(choke)=100*(10/15). So V(choke)=66.7 volts, or put another way, 66.7% of the RFI noise is dropped across the input of the power supply, where latter filtering stages need to deal with it.
With that said, we only need to find a suitable amount of ballast resistance in the power cord and branch circuit to dissipate as much RFI energy as we can. A common SJT extension cord made of the minimum AWG to bear the current demands of the device is all you really need. A heavier than necessary cord will exhibit less resistance that is an integral and essential part of the resistor/inductor filter. For most low to midrange power amplifiers, an 18AWG power cord will be your best overall choice. 18AWG is also the best choice for low-power consumption devices such as preamps and CD players.
Sure, a big fat 12AWG power cord looks impressive to the uninitiated, but if you understand the principle I outlined above, and your gear was endowed by its manufacturers with decent power supply input filtering, you now will understand that all that glitters is not gold in the world of power cords.
